One NHS orthodontist said if funding was doubled, lists in Carmarthenshire, Ceredigion and Pembrokeshire would take five years to clear. The Welsh government said it is working with health boards to improve services.
A spokeswoman for Hywel Dda University Health Board said: "Work continues to reduce the waiting times for treatment, however this is challenging given the number of referrals being received, but we have made it a priority to improve this for our patients."
David Howells, who runs the Pencastell Orthodontics, the only NHS Orthodontist in west Wales, told BBC Radio Wales said: "We've got very long waiting list for children.
"I gather that around 4,000 children are waiting for treatment and I think they're typically waiting about three years and I believe that might be the highest in Wales and perhaps the UK."
The health board has taken over managing the waiting list so all NHS cases are to referred to it and there is the capacity to only treat around 800 cases a year. As a result children are not being treated until well into their teens and, sometimes, in their 20s.
The board suggest that even if funding was doubled it would take at least five years to deal with the backlog of patients in the three counties.
The Welsh government say they have invested £700,000 "to improve the way patients are referred for dental and orthodontic treatment to make the system more efficient and reduce waiting times".